Processing of substrates, for example wafers intended to bear integrated circuits, is often carried out in a vacuum environment in which electron beam writing of patterns or substrates can be performed efficiently and with degrees of precision and fineness scale and detail beyond the capabilities of other writing techniques. Mass production of substrates with written patterns requires repeated movement of new substrates into the writing zone and removal of processed substrates from the zone, in each instance with relief and subsequent reinstatement of the vacuum. This consumes an appreciable proportion of the processing time for the substrates and thus increases the unit production cost. Substrate loading and unloading, in particular of a dedicated substrate holder, is generally carried out manually by a skilled operative, who must remove the holder from the machine, unload the processed substrate, fit a new substrate and reinsert the holder. In addition, fine adjustment of the substrate position in the holder, if needed, has to be carried out as a separate operation and requires the exercise of skill and knowledge.